In the development of proofs particularly colour proofs the photographic plate will pass through a development process in which the plate passes through in turn a developing section, a fixing section, a wash section and a drying section. In the developing and fixing sections a plate typically passes through a bath of process liquid. The process liquid in the respective bath will degenerate due to chemical reaction with the photographic material, and oxidation due to contact with the air. The process liquid in the bath will drop in level due to carry-over on the photographic materials and due to evaporation. It is therefore necessary to add fresh liquid to the bath. This addition thus depends on the amount of processed material but it will be time-dependent as well.
It is known to add fresh liquid to the bath by means of so-called chicken-feed bottles that supply liquid as the liquid level in the bath lowers due to carry-over, but also due to intentional removal of used liquid in order to cause spontaneous replacement by fresh liquid for reasons of oxidation compensation. Such removal occurs by opening a discharge valve in a bottom conduit of the tank, and the used liquid flows in a waste collection vessel. A disadvantage of this system is premature oxidation of procesing liquid in the bottle.
It is further known to carry out bath control by means of so-called cubitainers, i.e. a collapsable plastic bag in a rectangular cardboard box, which is in connection with a supply conduit through which liquid is withdrawn and pumped in the processing station under the control of a level sensor in the station, and a return conduit via which liquid can return to the cubitainer. The return conduit is normally closed by a discharge valve. This valve is opened for certain periods to withdraw an amount of liquid from the bath and automatically produce thereby its replacement by fresh liquid, in order to compensate for oxidation of the bath. It has been shown that time-controlled operation of this discharge valve, unlike the valve in a chicken-feed system comprising a waste collection vessel, does not produce true time-dependent discharge rates because the air pressure in the cubitainer fluctuates as a consequence of the air-tight connection of the supply and return conduits to the cubitainer and of the changing volume of the bag caused by a progressive removal of its contents to compensate for evaporation and carry-over of processing liquid from the bath.